What are your main programs that you use? What do you foresee yourself using this computer for?
I had an internship at a section of Activision (a large video game production company, if you don't know) for a while and every single computer was running 4 GB of RAM until I (personally) upgraded all the computers to 6 GB...and trust me...they use a ton of memory intensive applications.
It sounds like your coworker overkilled it without any knowledge of doing so. Yes, there will be extreme speed boosts from 2 GB to 4 GB. In cases of higher requirement programs, there will be a speed boost between 4 GB and 6 GB. If you are really using large 3d graphics, while watching a 1080p video, and....say....reading 40 million emails, then you might have a speed boost from 8 GB to 12 GB. See what I'm getting at? There is almost (read: zero) no need to have 12 GB of RAM. Your coworker could take out 2-4 GB of RAM and I guarantee you they will not see a speed decrease.
That said, they may have bought 2 sticks of 2 GB of RAM, decided to upgrade and found 2 sticks of 4 GB of RAM for a good price. It really depends on the cost...you might find 2 sticks of 4 GB is cheaper than 4 sticks of 2 GB. You might even find that 2 sticks of 4 GB is cheaper than 2 sticks of 2 GB! It really depends on the vendor, the brand, the speed, etc (obviously DDR vs DDR2 vs DDR3, but I assume you know that).
Again, I strongly suggest you decrease the amount you currently are going for. Obviously, getting 12 GB does not hurt your computer at all...it just hurts your wallet.
Another personal story: In my machine that I have 8 GB of DDR3 RAM, when I built it, I had 1 faulty stick. Now, I use my computer for a lot of cpu and memory intensive applications, and I still didn't detect that I had a faulty stick for about two weeks! Thats because the computer never (read: rarely) used it! For some tasks, the computer uses different, seemingly random memory locations, which is why it finally failed.
What are some programs you foresee using? What do you plan on doing with your computer that is CPU intensive?